1. Field of the Invention
The invention relates to a capacitor structure comprising a lower electrode layer disposed on a substrate and an upper electrode layer and a crystalline zinc oxide-containing dielectric layer interposed between the electrode layers. In addition a method for producing the capacitor structure and a use of the capacitor structure are specified.
2. Description of the Related Art
A capacitor structure having the described layer set-up is known from WO 2004/017063 A2 for instance. The capacitor structure (thin film capacitor) forms a piezo-acoustic thin film resonator (Film Bulk Acoustic Resonator, FBAR). The crystalline dielectric layer is a polycrystalline layer made of zinc oxide (ZnO). This layer forms a piezo-electric layer of the resonator. The electrode layers are made of platinum for instance. The electrode layers and the piezoelectric layer are arranged next to one another such that an electrical control of the electrode layers with an electrical alternating field results in the resonator vibrating at a resonance frequency. The resonance frequency of the vibration depends on the layer thickness of the layers of the capacitor structure. Which vibration mode (longitudinal vibration or sheer vibration) is induced depends on a crystal structure of the zinc oxide monocrystal and on a relative alignment of the zinc oxide monocrystal to the disposed electrical alternating field.
A vapor deposition method is carried out in order to produce the layers of the capacitor structure on a substrate, for instance a silicon substrate. In this process, the lower electrode layer, made of polycrystalline platinum for instance, is first deposited onto the silicon substrate. Zinc oxide is deposited onto the lower electrode layer made of platinum. Zinc oxide monocrystals grow at an (002)-orientation without additional measures. This means that the polar c-axis of the zinc oxide is oriented perpendicular to the substrate surface and/or to the electrode surface. This allows the resulting resonator to be optimally induced to longitudinal vibrations.
The known resonator is used to detect a substance of a fluid. To this end, the fluid is passed through to a surface segment of the resonator, with the substance to be detected being sorbed on the surface segment. The sorption leads to a change in the mass of the resonator results and thus to a change in the resonance frequency of the resonator.
If a fluid in the form of a liquid is to be examined, and the resonance frequency of the resonator is to be determined whilst the fluid is passed through, it is particularly advantageous to be able to induce the known resonator to sheer vibrations. Sheer vibrations are almost not attenuated by means of the fluid, thereby resulting in a relatively high quality resonator in comparison with the longitudinal vibrations and thus in a relatively high detectability for the substance of the fluid. The described production method results in an (002) orientation of the zinc oxide monocrystal. To achieve a resonator which can be induced to sheer vibrations, the zinc oxide monocrystals must grow at an angle. WO 2004/017063 A2 does not discuss how this can be achieved.